For vascular diseases (e.g. neurovascular, peripheral, or coronary), important diagnostic information is gained from the shape of the vascular system in observation and its flow dynamics. Such information can for example assess tumors, ischemic stenoses, and arterio-venous malformations. A system that allows the such diagnostics is known from the US 2003/0040669 A1, according to which the vessel geometry is reconstructed in three dimensions based on a first sequence of rotational projections. The flow characteristics are then determined from a second sequence of projections from a constant direction, and matched with the three-dimensional vessel model. Due to the necessary generation of two image sequences, the patient load with contrast agent and X-radiation is however rather high for this method. Furthermore, as the dynamic flow is observed in one fixed projection geometry, flow information is only available for a subset of vessels which run approximately parallel to the image plane without occlusion from other vessels, therefore not providing complete diagnostic information.